Gas separations utilizing solid adsorbent materials are well known, particularly those in which naturally occurring and synthetic zeolites are used. The synthetic zeolites are especially adapted to gas separations based on the size of the gas molecule.
A number of different synthetic zeolites have been formulated and are commercially available for gas separations. For example, such molecular sieve adsorbent materials are commonly used to remove high boiling impurities such as water vapor and carbon dioxide upstream of further processing. Such usage is common for pretreatment of natural gas feedstocks and precleanup of air prior to cryogenic separation. Other applications include the upgrading of refinery process streams such as recycle hydrogen streams. Additionally, other zeolites are commonly used in adsorption processes to separate air. Such zeolites are utilized in either pressure swing or temperature swing adsorption processes, although the pressure swing processes are generally preferred. These zeolites are typically nitrogen selective, that is, the nitrogen component of the air stream is adsorbed preferentially to the oxygen component. As a result, the nitrogen component is loaded onto the adsorbent bed whereas the oxygen component tends to remain in the gas phase. Although the zeolite molecular sieve adsorbent materials are effective materials for separating air, they have one significant drawback. That drawback is related to the fact that by nature of their nitrogen selectivity, it is the major component of air that is adsorbed rather than the minor oxygen component. Since air composition is nominally 78 percent nitrogen, nitrogen selectivity for the adsorbent results in large adsorbent material requirements for such a separation process. It would be advantageous, therefore, for a separation process to adsorb oxygen rather than nitrogen and thereby reduce the adsorbent material requirements.
The potential advantage of oxygen selective processes has been recognized and, for this purpose, oxygen selective carbon-type molecular sieve adsorbents have been made available. This type of adsorbent is rate selective, however. Consequently, these materials are necessarily used in nonequilibrium process cycles that maximize sorption rates of oxygen with respect to those of nitrogen. This, in turn, requires the use of rapid cycles, for example, cycles of about one minute in duration, which restricts cycle design to pressure swing adsorption processes and have relatively high power requirements.
Thus, there is a need for adsorbent processes which are both oxygen selective and non-rate selective.